2. Challenged with insufficient energy and material resources and
undesirable man made climate changes, science is searching for
new and innovative strategies to save, transfer, and store thermal
energy. Presently, one of the utmost intensively debated alterna-
tives is the so-called nanofluids. The suspension of metal or metal
oxide nanoparticles and CNTs in a base fluid is known as a nanofluid
(Choi and Eastman, 1995). Nanofluids are a new and promising
option as working fluids in thermosyphons, heat pipes, and solar
collectors (Buschmann, 2013; Said et al., 2015). Alumina is the most
cost effective and widely used material in the family of engineering
ceramics (Haddad et al., 2014). A large number of Al2O3-based
nanofluids are prepared by the two-step method using an ultra-
sonic vibrator, which results in non-stable nanofluids for a long
period of time. One of the utmost tasks to be accomplished is the
stability of nanofluids (Wei et al., 2009; Yu and Xie, 2012) for the
better thermal performance. Different approaches have been
selected by various authors for preparing stable suspensions using
different surfactant, optimizing the pH, temperature for numerous
nanoparticle based fluids, and by surface modification of the par-
ticles. Studies on using high pressure homogenizer for preparation
of Al2O3-based nanofluids are limited (Sridhara and Satapathy,
2011; Bobbo et al., 2012).
Significant enhancements in the thermal conductivity, and heat
transfer coefficient of working fluid, are known as the exceptional
physical effects of nanofluids. Solid phase metals have higher
thermal conductivity than the conventional fluids (Bejan and Kraus,
2003). Therefore, metal nanoparticles suspended in fluids are
anticipated to improve thermal conductivity compared to pure
fluids. Li and Peterson (Li and Peterson, 2006) dispersed oxide
nanoparticles (CuO and Al2O3 with 6% and 10% volume fractions) in
a liquid and reported enhancement in thermal conductivity at 34 C
by a factor of 1.52 and 1.3, respectively. Grimm (Grimm, 1993)
reported 100% improvement in the thermal conductivity of the
nanofluid for 0.5e10 wt. % of alumina nanoparticles suspended in
base fluid.
Recently, several studies have used nanofluids in solar collectors
to improve the thermal performance of the system. The effect of
using Al2O3 nanofluids in a flat plate solar collector as an absorbing
medium was studied by Tiwari et al. (2013). The effect of particle %
v/v and mass flow rate on the efficiency of the collector was also
considered in their study. The authors found a 31.64% improvement
in thermal efficiency for the 1.5% of Al2O3 nanofluid (Li and
Peterson, 2006). A similar experiment was done by Yousefi et al.
(2012a,b) to investigate the effect of Al2O3eH2O based working
fluid on the efficiency of a flat plate solar collector. Their result
showed that the efficiency of solar collector was increased by 28.3%,
while using 0.2% Al2O3 nanofluid instead of water as a working
fluid. Experimental investigations on the effect of Multi Walled
Carbon Nanotubes (MWCNTs) water nanofluids on the energy ef-
ficiency of flat plate solar collector by Yousefi et al. showed that the
improvement in the energy of the collector increased by 35%, using
(MWCNTs) water nanofluid as the working fluid (Yousefi et al.,
2012a; Yousefi et al., 2012b). Otanicar et al. (2010) experimentally
investigated different nanofluids, and the effect of these nanofluids
on the efficiency of a micro-solar thermal collector. An efficiency
enhancement of up to 5% was reported by them using nanofluid as
an absorption medium. Natarajan and Sathish used carbon nano-
tubes as a medium of heat transport to enhance the thermal con-
ductivity of base fluids, and reported improved efficiency of the
conventional solar water heater (Natarajan and Sathish, 2009).
Thus, it is important to improve the efficiency and performance of
the solar thermal systems. To the best of our knowledge, we found
that almost all of the previous works were directed on the appli-
cations of nanofluids in collectors and solar water heaters (Otanicar
Nomenclature
Ac collector area, m2
Cp specific heat, J/kg K
d diameter of pipe, m
DH hydraulic diameter, m
_Exin exergy rate at inlet, W
_Exout exergy rate at outlet, W
_Exdest rate of irreversibility, W
_Exheat exergy rate received from solar radiation, W
_Exwork exergy output rate from the system, W
_Exmass;in Exergy rate associated with mass at inlet, W
_Exmass;out exergy rate associated with mass at outlet, W
_EX;sun exergy rate, W
f friction factor
h specific enthalpy, J/kg
hin specific enthalpy at inlet, J/kg
hout specific enthalpy at outlet, J/kg
hnf heat transfer coefficient, W/m2
IT incident solar energy per unit area, W/m2
kp thermal conductivity of nanoparticle, W/m K
K loss coefficient (dimensionless)
ṁ mass flow rate, kg/s
V velocity of fluid, m/s
P pressure, Pa
J specific exergy, J/kg
q convective heat transfer rate, W
_Qo heat loss rate to the ambient, W
_Qs energy rate engrossed, W
_Qsun;in energy gain rate, W
R ideal gas constant, J KÀ1
molÀ1
Re Reynolds number (dimensionless)
sa entropy generation to surrounding, J/kg K
sin entropy generation at inlet, J/kg K
sout entropy generation at outlet, J/kg K
S absorbed irradiation, W/m2
_Sgen entropy generation rate, W/K
Ta ambient temperature, K
Ts sun temperature, K
Tsur surrounding/ambient temperature, K
TW wall temperature, K
_W work rate or power, W
Dl Length of pipe, m
Dh specific enthalpy change, J/kg
Dp pressure drop, Pa
Ds change in entropy generation, J/kg K
ɳo optical efficiency (dimensionless)
hII exergetic efficiency (dimensionless)
m viscosity, N s/m2
t transmittance
a absorptance
F nanoparticles volume fraction, %
r density, kg/m3
s overall entropy production, J/kg K
k thermal conductivity, W/m K
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3. et al., 2010; Yousefi et al., 2012a; Yousefi et al., 2012b; Mahian et al.,
2013; Tiwari et al., 2013). None of the above mentioned researchers
used a pH control for longer stability of Al2O3eH2O nanofluid.
Based on the above literature study, it has been found that au-
thors (Otanicar et al., 2010; Yousefi et al., 2012a; Yousefi et al.,
2012b; Said et al., 2013b; Tiwari et al., 2013) focused on the first
law efficiency of the solar collector operated with nanofluids or
MWCNTs. However, we have investigated exergy efficiency of the
solar collector operated with nanofluids. Moreover, first law effi-
ciency of the present investigation found to be higher compared to
the existing systems. Lastly, stability of the nanofluid obtained by
using a pH control approach was found to be better than the
existing literature.
2. Methodology
2.1. Solar water heater
Solar water heaters are the natural and carbon free process to
get hot water for many useful applications such as domestic, in-
dustrial and commercial applications. A solar water heater basically
consists of a collector and insulated storage; collector is used for
collecting solar radiation from sun and storage tank for storing the
hot water. Basic functioning of solar water heater is that solar en-
ergy from the sun incident on the absorber panel coated with
selected coating transfers the heat to the water flowing through the
tubes and the water passing through the tube gets heated which is
finally delivered to the storage tank. In general, the temperature of
water goes up to 60e70 C on a good sunny day and is useful for
many real life applications (Park et al., 2014).
Energy is based on the first law of thermodynamics and gives
the quantity of energy only. While exergy is based on the second
law of thermodynamics and represents the quality of energy and
involves the irreversibility while analysing system efficiency.
Exergy analysis identifies the causes, locations and magnitude of
the system inefficiencies and provides the true measure how a
system approaches to the ideal (Dincer and Rosen).
2.1.1. Energy analysis
Amount of energy conserved is overall the same but in different
forms of energy i.e. thermal, mechanical, internal, potential, kinetic
experience measurable changes. The general energy balance equa-
tion of the solar water heater (for a stationary process observed
through a control volume) may be given as below (Ceylan, 2012):
Qc ¼ Qw þ Qb þ QL (1)
where, Qc presents the absorbed energy by the collector, Qw pre-
sents the stored energy in the storage tank, Qb presents the stored
energy in the body and QL presents the lost energy (Chen et al.,
2009). The gained useful energy in the tank by water is:
Qw ¼ Qc À Qb À QL (2)
The energy storage in the tank is related to the mass and the
difference in temperature between the initial and final temperature
of water in the storage tank (Chen et al., 2009; Esen et al., 2009).
Qw ¼ mwCpw
Tf À Ti
(3)
Using the above equation, Qb and QL can be given by:
Qb ¼ mbCb
Tf À Ti
(4)
QL ¼ Ut
À
Tm;st À Ta
Á
(5)
where Ut is the coefficient of the total heat loss rate. For simplifi-
cation, the mean system temperature can be taken as the arith-
metic mean of the initial and final water temperature in the storage
tank as given below:
Tm;st ¼
Ti þ Tf
2
(6)
The thermal efficiency of the flat plate solar collector (h), is the
ratio of energy storage in the storage tank to the total solar radia-
tion on the collector, which can be expressed as (Al-Madani, 2006;
Roonprasang et al., 2008; Ceylan, 2012):
h ¼
mnf Cnf
Tf À Ti
IT Ap
(7)
2.1.2. Exergy analysis
Exergy is the maximum output that can be achieved relative to
the environment temperature. The general equation of the exergy
balance is (Suzuki, 1988; Farahat et al., 2009):
_Ein þ _Es þ _Eout þ _El þ _Ed ¼ 0 (8)
where _Ein is inlet exergy rate, _Es is stored exergy rate, _Eout is outlet
exergy rate, _El is leakage exergy rate, _Ed is destroyed exergy rate.
The inlet exergy rate measures the fluid flow and the absorbed
solar radiation rate. The inlet exergy rate with fluid flow can be
calculated by Farahat et al. (2009) and Bejan (1988):
_Ein;f ¼ _mCp
Tin À Ta À Ta ln
Tin
Ta
þ
_mDPin
r
(9)
where DPin is the pressure difference of the fluid with the sur-
roundings at entrance, r is fluid density.
The absorbed solar radiation exergy rate can be calculated as:
_Ein;Q ¼ hIT AP
1 À
Ta
Ts
(10)
where Ts is apparent sun temperature and equals to 75% of black-
body temperature of the sun (Bejan et al., 1981).
Total inlet exergy rate of the solar collector can be calculated as:
_Ein ¼ _Ein;f þ _Ein;Q (11)
At steady state conditions, where the fluid is flowing, the stored
exergy rate is zero.
_Es ¼ 0 (12)
When only the exergy rate of outlet fluid flow is considered, the
outlet exergy rate can be defined as (Kotas, 1995):
_Eout;f ¼ À _mCp
Tout À Ta À Ta ln
Tout
Ta
þ
_mDPout
r
(13)
The heat leakage from the absorber plate to the environment
can be defined as the leakage exergy rate and calculated as (Gupta
and Saha, 1990):
_El ¼ ÀUAPðTP À TaÞ
1 À
Ta
Tp
(14)
where the overall heat loss coefficient U is optimized at 4:6797 W
m2K
(Farahat et al., 2009).
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4. The destroyed exergy rate caused by the temperature difference
between the absorber plate surface and the sun can be expressed as
(Gupta and Saha, 1990):
_Ed;DTS
¼ ÀhIT APTa
1
Tp
À
1
Ts
(15)
The destroyed exergy rate by pressure drop is expressed by
Suzuki (1988):
_Ed;DP ¼ À
_mDP
r
Ta ln
Tout
Ta
ðTout À TinÞ
(16)
The destroyed exergy rate caused by the temperature difference
between the absorber plate surface and the agent fluid can be
calculated from Suzuki (1988):
_Ed;DTf
¼ À _mCpTa
ln
Tout
Tin
À
ðTout À TinÞ
Tp
(17)
So, the total destroyed exergy rate can be calculated from:
_Ed ¼ _Ed;DTS
þ _Ed;DP þ _Ed;DTf
(18)
The exergy destruction rate can also be expressed from:
_Ed ¼ Ta
_Sgen (19)
where _Sgen is the overall rate of entropy generation and can be
calculated from Bejan (1996a):
_Sgen ¼ _mCp ln
Tout
Tin
À
_QS
Ts
þ
_QO
Ta
(20)
Where _QS is solar energy absorbed (W) by the collector surface as
expressed by Esen (2008):
_QS ¼ IT ðtaÞAP (21)
And _QO is the heat loss to the environment (W),
_QO ¼ _QS À _mCPðTout À TinÞ (22)
Ultimately, combining all the expression above, the exergy ef-
ficiency equation of the solar collector can be analyzed (Farahat
et al., 2009):
hex ¼
_m Cp
Tout À Tin À Ta ln
Tout
Tin
À DP
r
!
IT Ap
1 À Ta
Ts
(23)
3. Experimental descriptions
3.1. Material
Commercial spherical shape Al2O3 nanopowder (Product ID:
718475) from Sigma Aldrich, Malaysia with 99.8% trace metal basis
and an average diameter of ~13 nm was used for the experimental
investigation. Reagent grade chemicals were used in the experi-
mental investigation. Distilled water was used as a base fluid while
hydrochloric acid (HCl-37%) was also used to maintain the pH of the
base fluid.
3.2. Preparation method and characterization
The previous decade has seen the speedy progress of nanofluid
science in diverse aspects, where the researchers concentrated
mostly on the improvement of heat transfer. Nevertheless
nanofluids preparation also deserves the similar devotion since the
final properties of nanofluids are reliant on the stability of the
dispersion (Haddad et al., 2014). The analytical analysis shows that
for a given particle size and zeta-potential, pH value of 7e9 gives a
higher stability ratio (W) and the same range of pH value is also
reported by Huang et al. (2009) for stability. Very few literature is
available on the stability of Al2O3 nanofluids using different pH
values (acidic or basic media) (Min et al., 2008; Sajid et al., 2014).
Moreover, Al2O3 is totally insoluble in water and it is amphoteric in
nature. To functionalize nanoparticles HCl solution was used. Ionic
strength of base-fluid can be adjusted tuning pH value (Min et al.,
2008). The above information suggests for a weak acidic base-
fluid to get aforementioned pH value. For reducing the aggrega-
tion and enhancing the dispersion behavior of Al2O3 nanoparticles
suspended in the base fluid, two notable methods were applied in
this study: (i) using pH 4 solution as the base fluid and (ii) using
high pressure Homogenizer (capacity up to 2000 bar) was used to
optimize the nanoparticles suspension (0.1% and 0.3%v/v) with pH
4 solution. A high pressure Homogenizer is an accepted machine for
dissolving the accumulated nanoparticle (Wei et al., 2009). Al2O3
nanoparticles with 0.1% v/v and 0.3% v/v were added to pH 4 so-
lution (base fluid) to obtain a homogeneously dispersed solution,
after passing the solution through several cycles for 30 min in a
high pressure Homogenizer. Addition of Al2O3 nanoparticles to the
pH 4 solution tends to increase the pH value ranging from 6 to 9
based on the % v/v (Haddad et al., 2014).
Field Emission Scanning Electron Microscopy (FESEM) and
Transmission Electron Microscope (TEM) were employed to
investigate the morphological characteristics of the nanoparticles.
A Zeta-seizer Nano ZS (Malvern) was used to obtain the average
diameter of the nanoparticles immersed in the base fluids. DLS
approach is used to give the hydrodynamic radius of particles in the
solution. The stability time of Al2O3eH2O is further supported by
visual images shown in Fig. 5. A KD2 Pro thermal property analyzer
(Decagon, USA) was employed to obtain thermal conductivity of the
nanofluids.
3.3. Experimental procedure
Fig. 1, shows the photographic image of the solar collector. Fig. 2,
shows the schematic presentation of the solar collector. The
experiment was performed at the University Malaya, Malaysia.
Table 2, shows the environmental and analytical conditions for the
flat plate solar collector. The properties Al2O3 used for thermal
performance calculations are presented in Table 1. For non-tracking
solar collection systems, the tilt angle has a predominant effect on
the quantity of energy that the system can intercept. An optimum
tilt angle of this flat plate solar collector is taken as 22 for the
maximum average daily radiation. An electric pump is used in this
solar collector system for the force convection heat transfer. The
heat generated from the collector cycle is absorbed by the tank,
with a capacity of 50 L, as shown in Fig. 2. A heat exchanger is used
outside the tank that transfers the heat load of the solar cycle to the
water. A flow meter is connected to the water pipe before the
electric pump (Fig. 2) to measure the flow of fluids. A simple valve is
used to control the mass flow rate of the working fluid in the solar
system. Five K type thermocouples were used to measure the fluid
temperatures at the entering and exit point of the solar collector as
shown in Fig. 2. Then these were connected to a 10 channel data
logger for data storage and analysis. A Li-COR Pyranometer (PY
82188) is used to measure the total solar radiation. The pressure
difference between the entry and exit point, was measured using a
pressure sensor. A PROVA (AV M-07) Anemometer is used to
measure the wind speed. All the data were then transferred from
data logger into the computer by an interface cable. Calibration of
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5. the entire system was carried out several times to obtain accurate
data. All the instruments used for this experiment were calibrated
according to the standards provided.
4. Testing method
An ASHARE Standard 93e2003 (Rojas et al., 2008) was used to
assess the thermal performance of the collector. The incident ra-
diation, ambient temperature inlet and outlet fluid temperatures
were measured and used for the thermal performance of the col-
lector. Thermo-physical properties of Al2O3 and base fluid are
presented in Table 1. Specifications and input parameters were
presented in Table 2. These values are needed for the calculation of
the first and second law efficiency analysis of the solar collector.
5. Uncertainty analysis
Uncertainty is needed to prove the accuracy of the experiments.
There are two kinds of error which could take place for the present
study. One group could come from the direct measurement pa-
rameters such as solar radiation flux (DGc),DT,DP and the second
group of errors could come from the indirect measurements, such
as energy and exergy efficiencies. The following relations can be
used based on the Luminosu and Fara (2005) method:
Dhex ¼
DI
:
_Exheat
þ
I
:
_Exheat
_Ex2
heat
(24)
and
Dhen ¼
Dqa
:
Gc
þ
qa
:
DGc
G2
c
(25)
where each error component can be evaluated through the
following relations:
DExheat ¼
DT
Ts
þ
TaDT
T2
s
!
AcðtaÞGc þ
1 À
Ta
Ts
AcðtaÞDGc (26)
DI
:
¼ TaDS
:
gen þ S
:
genDT (27)
DS
:
gen ¼
R ln
Pout
Pin
þ Cp ln
Tin
Tout
þ Cp
Tout þ Tin
Ta
Dm
:
þ GcAcðtaÞ
DT
T2
a
þ m
:
Cp
1
Tout
þ
1
Tin
þ
2
Ta
þ
ðTout þ TinÞ
T2
a
!
DT þ m
:
R
1
Pout
þ
1
Pin
DP
þ AcðtaÞ
1
Ts
þ
1
Ta
DGc
(28)
Dq
:
a ¼ Cp
Dm
:
ðTout þ TinÞ þ 2m
:
DT
Ac
!
(29)
For this experiment, K-type thermocouples with an accuracy of
±2.2 C or ±0.75%, a PROVA (AV MÀ07) anemometer with accuracy
Fig. 1. Photograph of the experimental setup: (a) Front view (b) Back view (c) Right side view (d) Left side view.
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6. ±3%, pressure transducer with ±0.3% (at ±25 C) accuracy (AZ
82100, digital manometer) and Li-COR Pyranometer (PY 82188)
with 1% accuracy were employed. Therefore, the maximum errors
for the indirect measuring of energy and exergy efficiencies were
estimated to be ±4.52% and ±3.75% using Eqs. (24) and (25).
6. Results and discussion
6.1. Stability and characterization of nanofluids
Fig. 3 displays the particle size distributions with respect to the
intensity acquired from the Zeta-seizer at dissimilar days. Fig. 4
presents SEM and TEM images. The high-pressure Homogenizer
was used to homogeneously disperse the well-isolated primary
particles. Therefore, better stability and size reduction of the
nanoparticles was obtained since the high pressure Homogenizer
was found to provide long-term stable and well-dispersed nano-
fluids and better particle breakdown.
Fig. 5 shows the visual appearances of the nanofluids with no
sign of aggregation for a period of a 30 days.
According to earlier studies (Vatanpour et al., 2011; Said et al.,
2013a; Said et al., 2013b), with higher concentration of nano-
particles in a solution, nanoparticles tend to agglomerate, therefore,
resulting in reduced stability of the nanofluids. It is witnessed that
the stability of the prepared nanofluid with a lower % v/v of
nanoparticles immersed into the base fluid is stable for a longer
period of time compared to the nanofluids with higher volume
fraction. Nanofluids with an average particle size of 106 nm and
with a high zeta potential value of 58.4 mV is presented in Table 3.
These values were obtained for more than a month period of time.
It was witnessed that with the growing volume fraction, the pH
shifts close to a basic value of (7e14). It was noted from the
experimental findings that pH 9 is the best value for a stable
solution.
6.2. Thermal performance
6.2.1. Solar radiation
Fig. 6 presents the recorded data for solar radiation on a clear
and cloudy day. This data is used for calculating the overall ener-
getic and exergetic efficiency of a flat plate solar collector, pre-
sented in Fig. 15.
6.2.2. Thermal conductivity
Since energy and exergy efficiencies are dependent on the
thermal conductivity of a nanofluid, details of the thermal
0
2
4
6
8
10
12
14
16
0.1 1 10 100 1000 10000
Intensity%
Size (nm)
1st Day
After 7 days
After 30 days
Fig. 3. Size presentation of watereAl2O3 0.1% nanofluids with pH 9.
Fig. 2. Schematic diagram of the experimental setup.
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7. conductivity analysis with the variation of temperature, concen-
trations are presented in this section.
Fig. 7 presents thermal conductivity of the experimental data
with the variation of % v/v of nanoparticles. Results of experimental
investigation were compared with the findings available in the
literature (Das et al., 2003; Yu and Choi, 2003; Xie et al., 2005).
Fig. 7 shows that with the growing volume fraction, the thermal
conductivity of the nanofluids increases. Since higher concentra-
tion of the nano-particles leads to higher thermal conductivity of
the working fluid, thus, resulting in a better heat transfer rate for
the solar collector.
Fig. 8 presents thermal conductivity variation with temperature
and comparison with the existing literature.
The experimental results presented in Fig. 8 agree well with the
literature (Chon and Kihm, 2005). Improved thermal conductivity
results, due to larger temperature differences, which result in
higher speed of molecules, and greater impacts between nano-
particles and the molecules of bulk liquid (Das et al., 2003; Chon
Fig. 4. (a) SEM of Al2O3 nanoparticles. (b) TEM images of Al2O3/water using control pH ¼ 9.
Fig. 5. Prepared Al2O3 nanofluid solutions (a) Samples on the first day of preparations (b) Samples after 30 days of preparations.
Table 1
Physical characteristics of Al2O3 and base fluid (Said et al., 2013a; Said et al., 2013b).
Particle base fluid Average particle size (nm) Actual density (kg/m3
) Cp(J/kg K) K (W/mK) Viscosity (m.Pa.s)
Al2O3 (gamma) 13 3960 773 40
Al2O3eH2O (0.1%v/v) 13 1020.5 3841.1
Al2O3eH2O (0.3%v/v) 13 1080.5 3159.3
Water 997.1 4179 0.605 0.89
Table 2
Specifications for the flat plate solar collector studied.
Parameters of collector Value
Dimension L2000 mm  W1000 mm  T80 mm
Aperture area 1.84 m2
Weight 36 kg
Frame Aluminum Alloy, Anodized
Working fluids in flow ducts Water and Al2O3 based nanofluid
Absorption area, Ap 1.84 m2
Wind speed 5 m/s
Collector tilt, bo 22
Absorption rate 0.94
Emittance 0.12
Heat transfer coefficient 4.398
Header material Copper TP2
Header tube size F22 mm  t0.6 mm(2pcs)
Riser tube material Copper TP2
Riser tube size F10 mm  t0.45 mm (8pcs)
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8. and Kihm, 2005; Li and Peterson, 2006; Mintsa et al., 2009). The
main mechanism behind the thermal conductivity improvement in
nanofluids is said to be as the stochastic motion of the nano-
particles. It was noted that with the increasing volume fraction, the
temperature difference between inlet and outlet was higher for
nanofluids operated collector compared to water as shown in
Fig. 14. This indicates that nanofluids can be used for higher heat
transfer rate. Higher temperatures of nanofluids results in more
active Brownian motion of nanoparticles. The Brownian motion is
dependent on the fluid temperature.
Fig. 9 illustrates the thermal conductivity improvement of
Al2O3ewater nanofluids at a volume fraction of 0.1%e0.3%,
respectively. The thermal conductivity improvement is directly
proportional to the % v/v and surges up to 6.8% with 0.3% v/v of
Al2O3, it improved from 2.46% to 6.80%. The experimental results
presented in Fig. 9 confirm the values achieved in other studies
(Vatanpour et al., 2011).
As it is presented in Figs. 7e9 thermal conductivity improves
with the growing % v/v as well as with increasing temperatures. An
increase in the heat loss may be faced in conventional collectors
with the increasing fluid temperature. However, the heat losses for
0.3% v/v nanofluid are lower in comparison with 0.1% v/v.
6.2.3. Efficiencies with respect to mass flow rate
The flat plate solar collector was tested at different mass flow
rates of 0.5, 1.0 and 1.5 kg/min. Each investigation was repeated for
several days, in order to achieve the best results with the least error.
A flow meter was used for controlling the mass flow rate. In this
part, both 0.1% and 0.3% v/v of nanoparticles with a controlled pH
were used as a working fluid. The collector efficiency at mass flow
rates of 0.5 kg/min, 1.0 kg/min and 1.5 kg/min were experimentally
investigated. The effect of mass flow rate on energy efficiency, en-
tropy generation, exergy destruction and exergy efficiency are
presented in Figs. 10e13, respectively. It is noticed from the energy
gain equation that the useful solar energy is directly proportional to
mass flow rate for a certain temperature increase. It was clear that
when the mass flow rate, and the inlet temperature increased; the
temperature difference reduced. According to the Figs., the effi-
ciency of the solar collector at low temperature differences de-
creases as the mass flow rate increases.
6.2.4. First law based efficiency
Table below shows the experimental data obtained from the
setup with water and with Al2O3eH2O (Table 4).
Energy efficiency has been estimated using Eq. (7) and input
data from Tables 1 and 2 presented in Fig. 10.
Fig. 10 demonstrates that the collector efficiency enhances with
the rising volume fraction. It has been observed that solar collector
operated with various concentrations of nanofluids has a higher
thermal efficiency than the solar collected operated with water as a
working fluid. The increase in the efficiency can be a result of the
increased thermal conductivity, which gives an improved convec-
tive heat transfer coefficient. An increase of 73.7% energy efficiency
was observed for 0.1% vol., whereas an increase of 83.51% energy
efficiency was observed for 0.3% vol. at the same flow rate (1.5 kg/
min). In case of water, a maximum energy efficiency of 42.07% is
observed for 0.5 kg/min, whereas an energy efficiency of 20.91% is
observed for 1.5 kg/min.
6.2.5. Entropy generation and exergy destruction of Al2O3 nanofluid
Heat transfer is an irreversible, non-equilibrium process from
the thermodynamic viewpoint. Entropy generation was considered
to be a measure of irreversibility (Onsager, 1931a, 1931b; Kreuzer,
1981). The irreversible losses affect the performance of the ther-
mal devices that results in an increased entropy and decreased
thermal efficiency. It is essential to calculate the entropy generation
Table 3
Zeta potential, particle size and pH values of Al2O3/water 13 nm particles suspended
in water.
Nominal particle
size
Zeta potential
(mV)
Particle size (nm)
from DLS using
high pressure
Homogenizer
pH
13 nm 58.4 106 9.0
13 nm 54.3 109 8.1
13 nm 49.5 123.9 7.0
13 nm 35.9 126.4 6.1
0
200
400
600
800
1000
1200
1400
9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00
Solarradiation(W/m²)
Time of day (hour)
Solar insolation on a clear day Solar insolation on a cloudy day
Fig. 6. Solar radiation on a clear day and cloudy day.
0.98
1.03
1.08
1.13
1.18
1.23
1.28
0.000 0.010 0.020 0.030 0.040 0.050 0.060
Keff/Kb
% v/v(φ)
Das et al. 2003
Xie et al. model 2005
Yu and Choi model 2003
Exp. Al2O3/water
Fig. 7. Models and investigational data on the thermal conductivity of Al2O3/water
nanofluids at changing volume fractions.
1.07
1.09
1.11
1.13
1.15
1.17
1.19
1.21
1.23
1.25
15 25 35 45 55 65 75
Thermalconductivityenhancment
(Knf/Kb)
Temperature (°C)
Al2O3, ~13nm (Experimental) C.H. Chon et al. 2005
Fig. 8. Thermal conductivity of nanofluids with respect to increasing temperature.
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9. or exergy destruction resulting from the heat transfer and viscous
friction as a function of the design variables selected for the optimal
analysis. Fig. 11 presents the entropy generation ( _Sgen) with regard
to mass flow rate and different volume concentrations of nano-
fluids. Entropy generation ( _Sgen) is calculated using Eq. (20) and
data from Tables 1 and 2
Fig. 11 shows the exergy destruction with respect to different %
v/v and mass flow rate. Reduced exergy destruction with the
increasing flow rate compared to water is observed for Al2O3eH2O,
which results in lower exergy destruction. Entropy generation
found to be much lower compared to water. The exergy destruction
(or irreversibility) rate, shown in Fig. 12, is calculated using Eq. (19)
and data from Tables 1 and 2
A decrease in the entropy generation is observed with the
increasing volume fraction. This happens because of the increasing
heat flux along the absorber plate, thus governing the irreversibility
turn out. As a result, enhanced thermal conductivity is obtained
with the increment in the volume fraction of the nanoparticles,
further proceeding to increase thermal conductance. Therefore, a
decline in the irreversibility is observed as a consequence of heat
transfer, which has a far greater effect compared to that of the
viscous effects of entropy generation. On the other hand, with the
growing nanoparticles volume fraction, the useful viscosity of
nanofluids is generated. The useful viscosity of nanofluids gives rise
to the fluid friction involvement in entropy generation, presented
in Fig.12. For a mass flow rate of 1.5 kg/min, nanofluid with 0.3% v/v
R² = 0.9965
0.99
1.00
1.01
1.02
1.03
1.04
1.05
1.06
1.07
1.08
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
Knf/Kb
volume fraction, φ
Experimental data Das et al. (2003) Linear (Experimental data)
Fig. 9. Thermal conductivity of Al2O3eH2O with respect to volume fraction.
20
30
40
50
60
70
80
90
0.5 0.7 0.9 1.1 1.3 1.5
Energyefficiency,%
Flow rate, kg/min
0.30% 0.10% Water
Fig. 10. The energy efficiency at different mass flow rates and different volume
fractions.
40
42
44
46
48
50
52
54
56
58
60
0.5 0.7 0.9 1.1 1.3 1.5
Entropygeneration,M/K
Flow rate, kg/min
0.30% 0.10% Water
Fig. 11. Entropy generation with respect to mass flow rate and volume fraction.
1300
1400
1500
1600
1700
1800
1900
0.5 0.7 0.9 1.1 1.3 1.5
Exergydestruction,W
Flow rate, kg/min
0.30% 0.10% Water
Fig. 12. Exergy destruction rate with respect to mass flow rate and volume fraction.
3
5
7
9
11
13
15
17
19
21
23
0.5 0.7 0.9 1.1 1.3 1.5
Exergyefficiency,%
Flow rate, kg/min
0.30% 0.10%
Water
Fig. 13. Exergy efficiency with respect to mass flow rate and volume fraction.
Fig. 14. Output temperature with respect to mass flow rates and volume fraction.
Z. Said et al. / Journal of Cleaner Production xxx (2015) 1e12 9
Please cite this article in press as: Said, Z., et al., Energy and exergy efficiency of a flat plate solar collector using pH treated Al2O3 nanofluid,
Journal of Cleaner Production (2015), http://dx.doi.org/10.1016/j.jclepro.2015.07.115
10. showed the least exergy destruction of 1432.37 W, followed by
1709.4 W for 0.1% v/v of nanofluid and 1855.7 W for water. It has
also been observed that with the rising mass flow rate of nano-
fluids, the efficiency of the solar collector also improves. Whereas,
with rising mass flow rate the temperature difference decreases.
This can be due to the reason that with lower mass flow rate
nanofluid has more time to absorb solar radiations and gain more
heat.
6.2.6. Second law/exergy based efficiency of Al2O3 nanofluid as
working fluid
Fig. 13 displays the behavior of the exergy efficiency as a func-
tion of the nanoparticles volume fraction and mass flow rate of the
fluid. Exergy efficiency was calculated using Eq. (23) and input
Tables 1 and 2
Based on Bejan's work (Bejan et al., 1981; Bejan 1996b, 1996c),
this analysis is carried out. The study is, however, used for flat plate
collectors as entropy generation minimization is vital to high
temperature systems. Maximization of the power output is the
same as the minimization of the entropy generation rate. It is
witnessed that the exergy efficiency, reduces with the growing % v/
v as well as with the growing mass flow rate. Al2O3eH2O nanofluid
shows higher values of efficiency compared to water base fluid. By
using Al2O3eH2O nanofluid in a solar collector as a working me-
dium, exergy efficiency can be enhanced. Al2O3eH2O can be a good
option as an absorbing medium because its exergy efficiency is
higher compared to water. Experimental results also reveal that by
suspending small amount of nanoparticles up to 0.1%, the exergy
efficiency could be enhanced by 20.3% compared the conventional
fluid.
6.2.7. Effect of Al2O3 nanofluid on output temperature
The effects of mass flow rate and volume fraction on the output
temperatures of nanofluids operated solar collector compared to
water as a working fluid are shown in Fig. 14.
As known the output temperature is one of the most effective
parameters that affects the energy efficiency of a flat plate solar
collector directly. It dramatically increased with rising output
temperature. Solar collectors, operated with nanofluids, provide
higher efficiency due to the higher output temperatures. The spe-
cific reason for higher output temperature is, the more nano-
particles in the base fluid. As we know, specific heat is defined as,
“The heat required to raise the temperature of a unit mass of a
substance by one unit of temperature.” It is clear from the definition
that any substance, which has a lower specific heat, should provide
higher temperature for equal heat flow.
6.2.8. Effect of overall first and second law efficiencies of Al2O3
nanofluid
The overall First and Second Law efficiencies with respect to
time is presented in Fig. 15. Using Eqs. (7) and (23), input data from
Tables 1 and 2 and solar radiation from Fig. 6, first law and second
law based thermal efficiencies has been estimated and presented in
Fig. 15.
Improved energetic and exergetic efficiencies are witnessed for
the studied nanofluids. From Figs. 6 and 15, it was noticed that the
maximum irreversibility occurs at noon, when solar radiation was
maximized; it decreased as solar radiation decreased. The tem-
perature difference between the collector and the ambient has an
ideal point for exergy efficiency, and a larger difference could result
in lower exergy efficiency. However, an increase in the temperature
difference decreases the energy efficiency due to the possibility of
more heat losses to ambient. According to the results from the
experiments, Al2O3eH2O nanofluid is found to be more appropriate
as a working medium for flat plate solar water heater than water.
7. Conclusions
An experimental study was carried out to assess the energetic
and exergetic efficiencies and the effect of pH control on
Al2O3eH2O nanofluid as a working medium in a flat plate solar
water heater. The effect of mass flow rate, nanoparticles volume
fraction, and the effect of pH on the energy and exergy efficiency of
the collector is examined. The obtained stability of nanofluid was
more than a month. The thermal conductivity improvement, ob-
tained by KD2 Pro, is directly proportional to the % v/v and surges
up to 6.8% with 0.3% v/v of Al2O3. The results obtained, showed that,
in contrast with water as the working medium, nanofluids increase
the first law efficiency by 83.5% for 0.3% v/v and 1.5 kg/min,
whereas the second law efficiency was enhanced by up to 20.3% for
9
11
13
15
17
19
21
23
10
20
30
40
50
60
70
80
8.45 9.45 10.45 11.45 12.45 13.45 14.45 15.45
Overallexergetic(2ndLaw)Efficiency(%)
Overallenergetic(1stLaw)Efficiency(%)
Operating Time
Overall Energetic (1st Law) Efficiency (%)
Overall exergetic (2nd Law ) Efficiency (%)
Fig. 15. Variation of overall energetic and exergetic efficiencies over time.
Table 4
Experimental data of the solar water heating system with and without nanofluids.
Local time (h) Volume concentration
(% v/v)
Solar radiation
(W/m2
)
Water temperature (
C) Mass flow
rate (kg/min)
Ambient temperature
(
C)
Wind velocity
(m/s)
Inlet Outlet
12:30 Water þ 0.1% of Al2O3 839.9 50.1 67.1 0.5 32.9 2.95
13:00 898.1 51.6 72.5 0.5 34.7 2.45
13:30 981.2 53.4 74.1 0.5 33.8 3.22
14:30 890.5 53.1 69.6 0.5 35.5 3.18
12:50 981.1 51.8 65.9 1.5 36.2 2.66
13:30 1066.0 52.5 66.7 1.5 36.4 2.75
14:00 1200.0 53.7 70.2 1.5 38.0 3.25
12:30 Water only 839.9 47.3 52.3 1.0 34.5 3.00
13:00 898.1 45.3 54.6 1.0 35.9 3.25
13:30 890.5 48.5 54.6 1.0 36.8 3.38
Z. Said et al. / Journal of Cleaner Production xxx (2015) 1e1210
Please cite this article in press as: Said, Z., et al., Energy and exergy efficiency of a flat plate solar collector using pH treated Al2O3 nanofluid,
Journal of Cleaner Production (2015), http://dx.doi.org/10.1016/j.jclepro.2015.07.115
11. 0.1% v/v and 1 kg/min. Increasing volume flow rate can increase the
efficiency of the system but the exergy efficiency will decrease. The
collector efficiency is directly proportional to the nanoparticle
concentration. It has been also observed that as the mass flow rate
of the nanofluids is increased, the efficiency of the solar collector
also improved. Whereas, with an increase in the mass flow rate the
temperature difference decreases. According to the results from the
experiments, Al2O3eH2O nanofluid is found to be more suitable as a
working medium for flat plate solar water heater than water.
To our knowledge, this is the first study, which is executed on
the energetic and exergetic efficiency analysis of a flat plate solar
collector using controlled pH for Al2O3eH2O, both theoretically and
experimentally. Accordingly, given the changing functioning and
atmospheric conditions that can occur in practice, the probability of
obtaining different results must be taken into account, in order to
ascertain the effect of the altered variables on the collector effi-
ciency and the characteristic parameters of the solar collector.
8. Future recommendations
For both scientific investigation and systems, stability of nano-
fluids suspension is an important challenge. Supplementary
emphasis should be given to the long term stability of nanofluids.
Additionally, the stability of nanofluids in the practical conditions,
should be given much more attention. Further research is needed
for thermal operations at higher temperatures, which would
tremendously beneficial for high-temperature solar energy ab-
sorption and high-temperature energy storage.
Therefore, there is a possibility for development in harvesting
the solar energy. It is recommended that the higher efficiency of the
collector can be achieved by reducing the losses and preventing the
settling of the nanoparticles. The size, form and % v/v of the
nanoparticles in the nanofluids directly influence the collector ef-
ficiency. More progress in the nanofluids properties and their
appropriate use in the solar energy harvesting will result in further
research in this field.
Acknowledgments
This research is supported by UM High Impact Research Grant
UM-MoE UM.C/HIR/MoE/ENG/40 from the Ministry of Education,
Malaysia.
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